1. Field of the Invention
The present invention relates to an electromagnetic coupler and a communication apparatus using the same that is suitable for a radio communication system for transmitting data using electrostatic field or induced electric field between communication devices located with a short distance.
2. Prior Art
As to conventional electromagnetic couplers, Japanese Patent No. 4345851 discloses one example of the conventional electromagnetic couplers. The electromagnetic coupler (radio frequency coupler) disclosed by Japanese Patent No. 4345851 is configured by connecting electrodes, a series inductor, and a parallel inductor provided on a flat plate via a radio frequency signal transmission line. In addition, the electromagnetic coupler is installed in a communication device such as transmitter or receiver. When the transmitter and the receiver are disposed such that respective electrodes of electromagnetic couplers in the transmitter and the receiver are facing to each other, in the case where a distance between the two electrodes is equal to or less than 2λ/15 of a wavelength λ of a used frequency, the two electrodes are coupled to each other by an electrostatic field component of a longitudinal wave, so that the two electrodes functions as one capacitor. As a result, such an arrangement totally functions as a band pass filter, so that it is possible to transmit the data with high efficiency between the two electromagnetic couplers. In the case where the distance between the two electrodes is 2λ/15 to 8 λ/15 of the wavelength λ of the used frequency, it is possible to transmit the data by using an induced electric field of the longitudinal wave.
On the other hand, when the distance between the two electrodes of the two electromagnetic couplers is a long distance which is greater than a predetermined value, the data transmission is impossible.
Accordingly, the electromagnetic coupler disclosed by Japanese Patent No. 4345851 is characterized in that other radio communication systems are not disturbed by an electromagnetic wave generated by the electromagnetic coupler, and the radio communication system using the communication device having the electromagnetic coupler is not interfered with the other radio communication systems. Based on these features, according to the radio communication system using the conventional electromagnetic coupler, it is possible to carry out the data transmission of large capacitance between the communication devices by UWB (Ultra Wide Band) communication method using wide band signal, with the use of the electrostatic field or induced electric field of the longitudinal wave over a short distance.
As described above, there is a problem in that the efficiency of the signal transmission is deteriorated when the electromagnetic coupler at one side does not match well with the electromagnetic coupler at an opposite side, although the data can be transmitted with high efficiency by realizing the band pass filter in the case where the distance between the electrodes of the two electromagnetic couplers is equal to or less than 2λ/15 of the wavelength λ of the used frequency. In other words, in the conventional electromagnetic coupler, the electrodes of the two electromagnetic couplers are coupled to each other by the electrostatic field component of the longitudinal wave, so that the two electrodes function as one capacitor. As a result, the band pass filter is realized by the series inductor and the parallel inductor connected to the two electrodes. However, for example, when this electromagnetic coupler is installed within the equipment and the radio communication is carried out, there is a cover of the equipment or the like including dielectric material between the electromagnetic couplers, so that a dielectric constant between the electromagnetic couplers varies due to the dielectric material. Therefore, a capacitance value of the capacitor provided between the electrodes of the two electromagnetic couplers varies, so that frequency characteristics of the band pass filter vary. In some cases, the data transmission characteristics in a desired frequency band may be deteriorated. For this case, even if the electromagnetic coupler is designed based on presumption that the dielectric constant varies, when the equipment carrying out the radio communication is different from the equipment including the designed electromagnetic coupler, the value of the dielectric constant between the electromagnetic couplers will be different from the designed value. Therefore, similar to the former example, the data transmission characteristics of the radio communication will be deteriorated.
In addition, when the distance between the electrodes of the two electromagnetic couplers is 2λ/15 to 8λ/15 of the wavelength λ of the used frequency, the data transmission is carried out by using the induced electric field component of the longitudinal wave. At this time, when location and surrounding environment of the two electromagnetic couplers are kept constant, the data transmission characteristics depend upon matching condition between the electromagnetic couplers and a feeding system. In other words, when the matching condition is good, intensity of signals transmitted from the electromagnetic coupler to a communication module including the feeding system increases. On the contrary, when the matching condition is bad, the intensity of the signals transmitted from the electromagnetic coupler to the communication module including the feeding system decreases. In the conventional devices, the electromagnetic coupler is designed in such a manner that the band pass filter is realized when the distance between the electromagnetic couplers is equal to or less than 2λ/15 of the wavelength λ of the used frequency. However, it is further necessary to consider the matching condition for the case where the distance between the electromagnetic couplers is 2λ/15 to 8λ/15 of the wavelength λ of the used frequency. Therefore, when the signal intensity is insufficient in the case where the distance between the electromagnetic couplers is 2λ/15 to 8λ/15, for example, it is necessary to re-design the device including a configuration for realizing the band pass filter in the case where the distance between the electromagnetic couplers is equal to or less than 2λ/15 of the wavelength λ of the used frequency, so that great care is required for designing the electromagnetic coupler. Further, when the used frequency band is a wide band, it is necessary to realize electromagnetic coupler adapted for a large number of frequencies for which the matching condition is suitable, so that troublesome work is required in designing.